1. Field of the Invention
Embodiments of the present invention are directed to apparatus and methods for in-well seismic sensing using fiber optic accelerometers. Embodiments of the present invention also are directed to apparatus and methods for in-well seismic sensing using optical hydrophones.
2. Description of the Related Art
Installing permanent seismic sensors in a wellbore to detect the movement of fluids over time in a surrounding reservoir would represent a significant advance in reservoir management, and is considered by many as a key element for instrumented oilfields. As companies focus more on maximizing recovery in existing and future fields, improved production and reservoir management is becoming increasingly important.
Conventional borehole seismic techniques, such as vertical seismic profiling (“VSP”), have been utilized for many years. VSP typically involves suspending an array of geophone sensors temporarily in the wellbore on a wireline, while actuating a seismic source at the surface. The sound waves generated by the source penetrate the earth and are reflected by rock and fluid interfaces, with the reflected signals being picked up by the geophones. As the surface source and/or geophone array is moved, a different set of reflections is received, helping to build up a seismic image of the subsurface in the vicinity of the well. Compared with normal seismic surveys which have both source and sensors at the surface, the result is a more detailed image of the reservoir, in which the distance that can be ‘seen’ from the wellbore is roughly determined geometrically by the depth of the well and the sensor locations. A variation on this theme is to locate the seismic source in a nearby wellbore rather than on the surface. Both of these methods are typically referred to as “active” monitoring.
Additional information can be gathered using conventional downhole seismology by “passive” monitoring in which no seismic source is used. Instead, the downhole sensors detect the natural seismicity of the reservoir and near-well region, for example, the “microseismic” signals emitted by rock formations as they are compacted and crack. Such passive monitoring has been used, for example, to verify the sinking of seabeds during gas extraction, and more generally to detect fluid movement through formations.
Another type of seismic downhole sensor which is currently utilized is a hydrophone. Hydrophones are used in a number of applications involving sensing acoustic pressures in fluidic media (e.g., liquid).
While seismic data can be actively or passively gathered using wireline-deployed geophones or hydrophones, seismic readings can only be taken periodically and require well intervention, perhaps requiring the cessation of fluid flow (i.e., production or injection) in the well. Moreover, while permanent geophone arrays have been deployed in relatively shallow wells, they are unsuitable for deeper deployments because their complexity and associated electronics cannot withstand the high temperatures and pressures encountered downhole for long periods of time. Downhole components must be adequately secured in place in the wellbore and must survive and consistently perform for as long as 20 years.
By contrast, optical sensors, such as optical fiber-based sensors, are significantly more resilient and have no moving parts or electronics to potentially be damaged by the downhole environment. Optical sensors have been deployed in wells to detect pressures, temperatures, distributed temperatures, flow rates and even phase fractions.
There is therefore a need for a seismic system which may be permanently deployed in a well which is more resilient and resistant to damage due to the downhole environment. There is also a need for a seismic system which is suitable for permanent, deep deployments. Additionally, there is a need for a seismic system deployable in a well which can withstand high temperatures and pressures downhole for long periods of time.